Technical Research on Intergranular Corrosion of TP321 Stainless Steel Seamless Pipe

Intergranular corrosion of 2205 duplex stainless steel pipe is a phenomenon of corrosion damage, manifested in the loss of binding force between grains, so that the strength of the material becomes worse. There are many different theories for the causes of intergranular corrosion, such as chromium depletion theory, grain boundary adsorption theory, precipitation-phase metastable theory, metastable phase dissolution theory, stress theory, precipitation phase morphology theory and corrosion electrochemical theory.Among them, Chromium-depleted theory is the earliest proposed and widely accepted theory. For 18-8 type austenitic stainless steel, the crystal lattice at the grain boundary is incomplete, which is conducive to the diffusion of metal atoms; Cr at the grain boundary and its neighboring regions will occur due to the precipitation of carbide Cr23C6 at the grain boundary. The phenomenon of leanness causes chromium-depleted regions around the grain boundary. When the Cr content is reduced to about 12%, corrosion occurs along the grain boundary of the material in certain corrosive media, and the bonding force between the grains loses, and grain boundary corrosion occurs. phenomenon.TP321 stainless steel (UNSS32168) is based on TP304 stainless steel with Ti added to enhance its resistance to intergranular corrosion and high temperature resistance. The principle is to form a stable MC type carbide TiC, which is measured by carbide formation free enthalpy change. TiC is much more stable than chromium carbide, reducing the formation of chromium carbide.In Europe and the United States and other developed areas market, TP321 stainless steel seamless steel pipe has been gradually replaced by TP304L, TP316L and other low-carbon, ultra-low carbon stainless steel seamless pipe; but in China, TP321 stainless steel seamless pipe demand is still large, according to international stainless steel According to the data released by the Forum (ISSF), in 2012, the apparent consumption of TP321 stainless steel seamless pipes in China was around 100,000 tons. Due to the limitation of production technology and inspection conditions, the pass rate of intergranular corrosion resistance of TP321 stainless steel seamless pipes produced in China is not high, and the pass rate of one inspection is about 80%. Shanxi Taigang Stainless Steel Pipe Co., Ltd. has been put into production since 2009, TP321 stainless steel seamless pipe production accounts for about 35% of total output, and the first-pass rate of intergranular corrosion inspection is about 70%, which is far below the level of other stainless steel products (?95% ). In order to increase the finished product rate and better meet the needs of users, the company's technical personnel conducted a technical research on the intergranular corrosion resistance of the TP321 stainless steel seamless pipe. The goal is to increase the qualification rate of one inspection to more than 95% and reach the Japan Sumitomo Metal Company. And other advanced foreign manufacturers.At present, the domestic production of TP321 stainless steel seamless steel pipes mostly uses perforation? cold rolling (pulling)? heat treatment? straightening? pickling? inspection? packaging production methods. After cold deformation of the steel pipe, solution heat treatment is used to remove the deformation stress and improve the organization, that is, the steel pipe is heated to a temperature above the austenite carbon saturation curve, so that the carbide is fully dissolved into the solid solution and then rapidly cooled, and the high temperature structure is at room temperature Fix it and get carbon supersaturated solid solution. Through benchmarking activities, the production process is analyzed, starting from aspects such as chemical composition control, heat treatment system adjustment, degreasing process optimization, etc., making the TP321 stainless steel seamless steel pipe's intergranular corrosion qualified rate of one inspection stable at more than 95%, achieving the target of research.. Specifically in:(1) Through the chemical composition design, adjust the ratio of C, Cr, Ni, Ti and other elements to optimize the intergranular corrosion resistance of 321 stainless steel pipe seamless pipe;(2) When TP321 stainless steel seamless seam steel pipe is treated by solution heat treatment, the reducing atmosphere in the furnace can easily cause carbon addition on the surface of the steel pipe. The weak oxidizing atmosphere has good quality on the surface of the steel pipe, eliminates the carbon addition factor, and saves energy.(3) TP321 stainless steel seamless steel tube after degreasing after cold rolling is not clean, has a greater influence on the surface quality of the steel tube after heat treatment, and also has an adverse effect on the corrosion resistance; by improving the degreasing method, the resistance to intergranular corrosion of the steel tube can be effectively improved;(4) The solution heat treatment heat preservation temperature is set to 1050 °C, which is beneficial to the intergranular corrosion resistance of TP321 stainless steel seamless steel tubes;(5) If TP321 stainless steel seamless pipe is used within the sensitive temperature range (450-900°C) and there is a strong corrosive medium in the environment, the steel pipe should be stabilized.

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